Biggest "bang for the buck" audio gear you have owned(in your history)?

This is very subjective, and your opinion on your own systems/items is all that really counts.

From the beginning to now, these would fit that category for me: (in no order)

Original Large Advents
Dahlquist DQ-10
Conrad Johnson ART pre.
Original Monster interconnects and speaker wires.
Nakamichi SR-3a
AR Turntable
Audio Research LS-12
Magnepan 1.6
Magnepan 20
Wireworld Gold Eclipse III
Audio Technica Electret Condenser headphones 
Magnum Dynalab Etude and 108 tuners
Magnum Dynalab Receiver (original)
And in those "early days" special pressings and imported Jazz albums. 

There may be others I am forgetting, but these were very good items along my journey. 

@ tooblue

B&K ST140 power amps....

I knew the B&K guys back in the mid to late 80s, and owned a Discrete Technology LS-2, which was a modified ST-140. I was thrilled to find one of the latest versions of ST140 power amp for $140 recently. Mint shape....added it to my son’s system as a grad school graduation gift. Great amp, great guys. Was sorry to see them pack it in.
 B and K  - Pro10 MC. 
Old pair realistic 12 inch floor speakers, been in garage for 20+ years, dry rot surrounds, but low volume, still sound good.
must be over 30 years old easy!
    Who made the realistic speakers? Mach 1, 2 etc etc. 
they were good speakers, they bested many of the so called upper echelon loudspeakers of the 80s’. 

Ilumnia Magister MK2 speakers ( the best !!!)
VAC phi 300.1
Metronome Technologie C6
Daniel Hertz M6L 
Audio Note CD 2/two
Moon streamer (drive)
Audio Note M6
Ear audio by Paravicini !!!
Wavac M 805

Never again:
Musical Fidelity 
Unison research …
Ikea Lack is best deal in audio I know. I have owned five racks, including fairly expensive to me Finite Elemente and SRA. The lack makes high performance shelving accessible for everyone.

Another one is 47 labs OTA cable "kits" with the caveat that need stay under about 2 feet in length to avoid frequency imbalance. For the price, though, their tone is great, they and crush my drawer of high profile branded cables for similar cost.   
This is a great question; one that touches the nostalgia vibe. Not sure this qualifies as best bang for the buck, per se, because, as an impoverished student in 1972, I should have spent this money on my education, instead of my first serious sound system. However, love of music made me do it! So, for me at the time, I guess it was money well spent because this sound system, these components, served me very well for many, many years. The scene of the crime was Tech HiFi in Cambridge, MA. I parted with $350 for a Sansui 2000X and, as best I can recall, around $160 for a Phillips 212 with Shure cartridge. In today dollars, that would be approximately $3,233. I skimped on the speakers, a pair of Studiocraft bookshelves. Can't remember what I paid for those but do remember the entire tab coming to less than $600. A year later, because of the trade-up incentive Tech HiFi offered in those days, I upgraded the Studiocraft with a pair of Ohm C. I upgraded the Sansui (still have it) and the Ohm C in 2001 and the Phillips in 2018. The Ohm C had been re-foamed once and the Philips had a minor repair in the mid 1990's. The Sansui has never been repaired or refurbished but its capacitors & transistors have given their best. How's that for value and solid engineering? Many audiophile friends with systems twice and three times the cost of mine often scratched their heads when visiting, wondering why this system sounded better than theirs. As our parents were fond of saying: "They built 'em good in those days!"